1. Field of the Invention
The present invention relates to a method of making semiconductor wafers, and more particularly of making monocrystalline silicon wafers for use in fabrication of semiconductor devices.
2. Description of the Prior Art
In general, a method of making semiconductor wafers comprises the steps of slicing a monocrystalline ingot pulled in a monocrystalline ingot puller to obtain thin disk-shaped wafers; chamfering the wafer periphery to prevent cracks in and chipping off of the thus sliced wafer in the following step; lapping the chamfered wafer to flatten the surfaces; etching off the work damage retained in the lapped wafer; polishing the etched wafer across one of the surfaces; and cleaning the polished wafer to remove the residual of a polishing slurry and the other foreign substances on the wafer.
It is known that there are available an acid etching that uses an acid etching solution such as a mixed acid and an alkaline etching that uses an alkaline solution such as a sodium hydroxide solution as the etching treatment of the above mentioned etching.
In the acid etching, a higher etching rate is attained and there is observed a finer surface texture under 0.6 .mu.m in roughness (the peak-to-valley height) and under 10 .mu.m roughness spacing as shown in FIG. 4, whereas in the alkaline etching a lower etching rate is attained and there is observed a rougher surface texture as large as 10 to 20 .mu.m in roughness spacing (and sometimes over 1.5 .mu.m in the peak-to-valley height).
On the surface of the acid-etched wafer, a waviness (hereinafter referred to an ORP-observed irregularity) having a waviness spacing of 5 to 20 mm and the peak-to-valley height under 0.2 .mu.m is produced, so that the flatness of such wafer is poor as shown in FIG. 6. On the other hand, on the surface of the alkaline etched wafer, none of the above described ORP-observed irregularity is produced and therefore the flatness of the wafer surface is kept during the etching in a rather good condition as shown in FIG. 6. In the FIG. 6 the ordinate axis in the rectangular coordinate system represents the local surface height measured with respect to a reference plane as the index of the flatness.
The ORP is the short for Optical Reflection Projector which is designed on the basis of the widely known Magic Mirror Principle or Magic Mirror Topography. Incident beams of light from a point source on a mirror-polished surface having a waviness are reflected back to an opposing imaging plane on which plane a variety of local illuminances are observed, a lighter portion corresponding to a recess on the waved mirror-polished surface.
A semiconductor wafer made by passing through the various steps above mentioned has the problems below described in the electronic device fabrication process where the semiconductor wafer is in use therefor, since the backside surface remains as it is etched.
The front surface does not cause any problem because after the wafer was etched, it is mirror-polished in the following polishing step and there is no need for holding it by suction on a stage in the electronic device fabrication process. But when the back surface acid-etched in the etching step is sucked to hold on a stage in the optical lithography step, the ORP-observed irregularity produced on the back surface is transferred on the front surface as it is, so that the ORP-observed irregularity reduces the resolving power of an exposure system, and thereby causes the yield of an electronic device fabrication therefrom to lower.
Furthermore, when the alkaline-etched back surface is sucked on a stage, particles are produced because sharp tips in the surface irregularity of a large surface roughness collapse by chipping off and that with a large number of the particles, resulting in the problem that the yield of an electronic device fabrication is reduced.
Shown in FIG. 7 are the numbers of particles being produced by holding on a stage by suction a variety of wafers that comprise those as finished by alkaline etching, acid etching and mirror-polishing respectively. The procedures or principle of the counting the number of the produced particles is described here. First of all, a reference polished wafer is counted across the polished surface as regards particles larger than 0.1 .mu.m across; then the reference is superposed on a sample wafer, the reference side down, where the measuring surface of the sample is posed to face upward, next the reference wafer is pushed downward to press the sample at a pressure of 1 Kgf/cm.sup.2 for 1 min for example, and lastly the reference wafer is removed to be ready for measurement of transferred particles on its reference side. The reference wafer is again counted in the same way as for the first time and the increase of the particle count across the surface is evaluated as "Number of Particles".
An alkaline-etched wafer is counted 4000.about.5000 particles and an acid-etched wafer about 2000 particles while a wafer as finished by both-sided polishing shows almost no particles produced through the above mentioned procedures.
Consequently, with a wafer as finished by both-sided polishing, particle generation in handling the wafer is suppressed because of the absence of large surface irregularities on the back surface and what's more, a better flatness is achieved due to no existence of ORP-observed irregularity, so that the above-mentioned problems are solved.
According to the above mentioned both-sided polishing, however, the disadvantages entail that the wafer detecting sensor that is generally accepted and which is designed to detect wafers by detecting the reflected light does not work on the back side as well as a second polishing step is added to increase the cost the more.